I was saved from buying a Saab several years back. Clean inside and out, drove well enough. Took it to a mechanic who was able to identify several issues and predicted that codes had been cleared but the check engine light would come back on on my drive back to the owner. He was right.

Earlier this year, my brother bought a super clean Toyota 4Runner. At least, he thought it was clean—and it was a Toyota, right? So no need to take it to a mechanic? Wrong. Check engine light came on and off intermittently after he bought the car, and it ended up costing him something like $4k in engine repairs and he was without the vehicle for almost a month.

Most shops will charge something like $50-80 to look a car over, plug their code reader in, etc. Well worth it, IMO.